NEAR AJDABIYA, LIBYA - Buoyed by U.S. and allied airstrikes that relieved a siege of Benghazi, Libyan rebels launched an offensive early Monday aimed at retaking the strategic city of Ajdabiya, as Western warplanes continued pounding forces loyal to longtime leader Moammar Gaddafi.

Explosions and plumes of smoke marked the scene of the strikes against Gaddafi's forces in Ajdabiya, about 100 miles south of the rebels' de facto capital of Benghazi, and advancing rebels cheered when Western warplanes flew overhead. The rebels by Monday had regained control of Zuwaytinah, an oil terminal about 16 miles northwest of Ajdabiya that had been captured by loyalist forces last week, news agencies reported

Speaking at a news conference Monday in Santiago, Chile, the latest stop on a South and Central American trip, President Obama defended his decision to deploy the U.S. military in Libya, saying the international community couldn't "simply stand by with empty words" in the wake of continued attacks by the Libyan government on its rebels.

Despite the airstrikes, however, Gaddafi's forces were digging in outside Ajdabiya, which straddles highways that go north to Benghazi and east across the desert to Tobruk.

From a point about five miles from the northern entrance to Ajdabiya, rebels jumped into dozens of vehicles and made a massive push toward the city Monday when they heard jets in the air and the sounds of bombardment. But after about half a mile, the rebels came under fire from loyalist tank and mortar shelling and promptly turned back.

Afterward, rebel commanders said they plan to wait for more allied airstrikes against Gaddafi's forces before pushing forward again.

Some Gaddafi opponents were concerned that the coalition has primarily focused on Benghazi even though rebels are holding out against the government in other areas, notably the western city of Misurata, Libya's third largest, where Gaddafi's troops launched a major assault on the eve of the U.N. vote authorizing the use of force.

Gaddafi forces reportedly made further advances on Monday, with government tanks rolling into the city and attempting to fight their way to the symbolic central square. At least 14 people were killed and more than 100 were injured, said a doctor at the hospital in Misurata who spoke on condition of anonymity because he fears the consequences of being named.

He said rebel fighters managed to hold onto the square, but Libyan forces had secured control of several neighborhoods, and had positioned snipers on buildings on a main road leading through the city, where they were shooting at anyone who moved. Tanks and artillery continued to pound rebel held neighborhoods throughout the day. Overnight, a helicopter attacked the antenna of the local radio station.

"What no-fly zone?" he asked. "I am sure the U.N. has forgotten us."

Libyan government spokesman Moussa Ibrahim said that Misurata had been "liberated", but that pockets of what he called Islamic extremists who are prepared to die for their cause are continuing to resist government forces.

He said coalition forces had struck a harbor 27 kilometers west of Tripoli on Monday, and the airports in Gaddafi's hometown of Sirte and in the southern town of Sabha in earlier strikes. He claimed all these targets were civilian facilities and that "many" civilians had been killed. But he had no update to the original number of 48 civilians killed in the first night of bombing issued by the government.